This invention relates to an annulus fluorescent lamp that comprises a plurality of fluorescent tubes having different diameters and communicated with each other at a bridge portion.
It is known that if a temperature of the lowest temperature point of a fluorescent tube rises along with an ambient temperature during operation, a mercury vapor pressure in the fluorescent tube rises and lamp luminosity and luminescence efficiency decrease. Especially, in the annulus fluorescent tube, an electrode portion at an end of the tube and the lowest temperature portion at another end of the tube are close to each other, so that heat generated at the electrode portion is easily transferred to the lowest temperature portion and raises the temperature at the lowest temperature portion.
To solve the above-mentioned problem, it is proposed to form through holes in a mouthpiece that covers both ends of the fluorescent tube for controlling the temperature of the lowest temperature portion during operation and improving the luminosity as disclosed in Japanese laid-open patent application (Tokukai-Hei) 8-241669.
However, the above-mentioned configuration has a problem. If a metal piece or other small object enters inside of the mouthpiece via the through hole, power lines such as exposed electric wires connected to the electrodes may be short-circuited. Even if the through holes are small independent holes as disclosed in the above-mentioned document, such exposure of the power lines cannot be prevented completely.
Particularly, a double annulus fluorescent lamp needs such through holes in the mouthpiece for ventilation to suppress a temperature rise at the lowest temperature portion, since it is operated under a large load condition and has a tendency that the temperature at the lowest temperature portion rises easily. Therefore, the problem of the exposed power lines, that is a lack of insulation, may possibly occur.